It is well known that acrylic fibrous materials, when subjected to heat, undergo a thermal stabilization reaction wherein the fibrous material is transformed to a black form which is non-burning when subjected to an ordinary match flame.
Such modification generally has been accomplished by heating the acrylic fibrous material in an oxygen-containing atmosphere. It is believed that the resulting thermal stabilization reaction involves (1) an oxidative crosslinking reaction of adjoining molecules, (2) a cyclization reaction of pendant nitrile groups to a condensed dihydropyridine structure, and (3) a dehydrogenation reaction. The cyclization reaction is exothermic in nature and must be controlled if the fibrous configuration of the acrylic polymer undergoing stabilization is to be preserved. The thermal stabilization reaction heretofore has generally been believed to be diffusion controlled and to require considerable time for oxygen to enter the interior portions of the fiber.
On a commercial scale, the thermal stabilization reaction commonly is carried out on a continuous basis with a continuous length of a multifilament acrylic fibrous material being passed in the direction of its length through a thermal stabilization zone which is provided with a heated gaseous atmosphere. The movement of the continuous length of acrylic fibrous material through the stabilization zone containing the heated gaseous atmosphere may be directed by rollers situated therein. The continuously moving length of acrylic fibrous material must be heated in air at approximately 250.degree. C. for two to three hours to completely stabilize the material. This time consuming thermal stabilization greatly increases the eventual cost of the carbon fiber produced from the acrylic fibrous material.
Representative United States patents which concern the thermal stabilization of an acrylic fibrous material include: U.S. Pat. Nos. 3,285,696; 3,539,295; 3,699,210; 3,826,611; 3,961,888; 4,186,179; and Re. No. 30,414. Since the thermal stabilization reaction has tended to be unduly time consuming, various routes have been proposed to expedite the desired reaction through some form of catalysis and/or chemical modification of the acrylic fibrous precursors. See, for instance, the following United States patents which are representative of this approach: U.S. Pat. Nos. 3,592,595; 3,650,668; 3,656,882; 3,656,883; 3,708,326; 3,729,549; 3,813,219; 3,820,951; 3,850,876; 3,923,950; 4,002,426; and 4,004,053.
The resulting acrylic fibrous materials can be used in the formation of non-burning fabrics. Alternatively, the stabilized acrylic fibrous materials can be used as precursors in processes for the formation of carbon or graphitic carbon fibers. U.S. Pat. Nos. 3,775,520 and 3,954,950 disclose representative overall processes for forming carbon fibers beginning with an acrylic precursor.
There has remained a need for a simple expeditious process for the formation of thermally stabilized acrylic fibrous materials. Such need is particularly acute in the overall context of carbon fiber production since the carbonization or carbonization and graphitization portions of the overall process commonly require a considerably lesser residence time than the initial thermal stabilization portion of the process. Accordingly, heretofore it has been essential to provide extremely large ovens in order to accomodate the acrylic fibrous material undergoing thermal stabilization if the entire process is carried out on a continuous basis with the fibrous material passing directly from the stabilization zone to the carbonization zone.
It has heretofore been proposed to apply ionizing radiation to acrylonitrile monomer, at very low temperatures, prior to the polymerization and spinning of the polymeric acrylic fibers which are subsequently thermally stabilized and carbonized. See, for example, U.S. Pat. No. 3,681,023 and U.K. Pat. No. 1,256,608.
It has also heretofore been proposed to irradiate fibers of homopolymers of polyacrylonitrile or copolymers of polyacrylonitrile with 1% and 5% methylacrylate with gamma radiation from a cobalt-60 source. See, Simitzis, J., "The Effect of .gamma.-Irradiation on the Pyrolysis Behavior of Polyacrylonitrile Fibers", Atomkernenergie Kerntechnik 33 [1], 52-56 (1979); and Simitzis, J., "On the Properties and Pyrolysing Behavior of .gamma.-Irradiated Polyacrylonitrile Fibers", Atomkernenergie Kerntechnik, 38 [3], 205-210 (1981).
However, at the disclosed dose rate of 0.184 megarads per hour, the acrylic fibers involved in the Simitzis studies must have residence times of exposure to gamma radiation on the order of 70 to 500 hours to provide energy absorption of from 13 to 90 megarads. Such residence times would severely lengthen the conversion of acrylic fibrous material to carbon fibers, and make the Simitzis process not commercially viable.
Further, while Simitzis recognizes that prior gamma irradiation of the fibers accelerates the subsequent oxidation, the indicated accelerated stabilization times are still on the order of 1.5 hours at 255.degree. C.
Therefore, it is an object of the present invention to provide an improved process for the thermal stabilization of acrylic fibrous materials.
It is an object of the present invention to provide an improved process for the thermal stabilization of an acrylic fibrous material which surprisingly can be carried out on an expeditious basis.
It is an object of the present invention to provide an improved process for the thermal stabilization of an acrylic fibrous material which can be carried out without the excessive usage of energy as commonly required in the prior art.
It is an object of the present invention to provide an improved process for the thermal stabilization of an acrylic fibrous material wherein oxygen readily enters the interior of the acrylic fibrous material without any substantial formation of a diffusion limiting skin on the outer surfaces of the fibers during the course of the thermal stabilization reaction.
It is another object of the present invention to provide an efficient process for the stabilization of an acrylic fibrous material immediately prior to the carbonization or carbonization and graphitization of the same.
It is an object of the present invention to produce an improved process for the stabilization of an acrylic fibrous material wherein thermal stabilization is expedited (i.e., occurs in 10 to 30 minutes).
It is a further object of the present invention to provide an improved process for the stabilization of an acrylic fibrous material that results in a significant reduction in the weight loss suffered by the acrylic fibrous material upon carbonization.
It is another object of the present invention to provide an improved process for the stabilization of an acrylic fibrous material wherein said material may be introduced into the stabilization oven at a substantially higher temperature than commercially utilized in the prior art, thus further accelerating thermal stabilization.
These and other objects of the invention, as well as its scope, nature, and utilization will be apparent to those skilled in the art from the following detailed description and appended claims.